feat: serialize nillable copmlex types as nil

[mirkoSekulic]

Description

Serialize nillable complex types as nil if the value is set to null.

Related Issue(s)

• "nil=true" in XML disapears for elements of type xs:date app-lib-dotnet#911

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.56%. Comparing base (276f5ef) to head (5c1a432).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14320   +/-   ##
=======================================
  Coverage   95.56%   95.56%           
=======================================
  Files        1837     1837           
  Lines       23856    23856           
  Branches     2754     2754           
=======================================
  Hits        22798    22798           
  Misses        801      801           
  Partials      257      257           

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